The prior art teaches a variety of methods for applying solder to printed circuitry substrates as well as various ways to control the amount of solder deposited at any given location on the substrate. Conventionally, solder paste is deposited through a mask or screen, that is, a metal stencil with apertures arranged in a pattern corresponding to the desired solder locations on the substrate. Normally, the thickness of solder deposited is controlled as a function of the mask thickness, where the thicker the mask the more solder paste is deposited.
U.S. Pat. No. 3,735,730 to Mitter discloses apparatus for applying a flowable medium to a carrier. The apparatus is provided with a spreading element for applying the flowable medium evenly at all points and the pressure of which upon the exposed surface and the density of the applied flowable medium can be varied continuously during the operation of the apparatus.
When it is desired to mount both leaded through hole and surface mount components to a single printed circuit board, adjustments must be made in conventional prior art techniques for applying solder. The amount of solder paste deposited at each component site is critical, and the amount required in the through holes varies from that required at a surface mount land. The conventional method for attaching both pin in hole and surface mount components uses two solder applications. The solder paste is first applied to the surface mount sites by screen printing, then reflowed. This step is followed by a second step using wave soldering to apply solder to the pin in hole components.
Another prior art method for attaching leaded through hole and surface mount components to a single printed circuit board is taught in U.S. Pat. No. 4,515,304 to Berger wherein the leaded through hole components are first placed on the printed circuit board. The lead ends are crimped to hold them in place while solder paste is applied, using individual applicator nozzles, to the lead ends and to solder pads for the surface mount components. Steps of inserting, crimping, turning the board and then applying solder are required. Then Surface mount components may then be placed on the second side of the printed circuit board and the whole reflow soldered.